libhb: add generic code for converting NAL
unit bitstreams (H.264, HEVC) to and from
Annex B format.

Only used by QSV for now, but could
always come in handy down the road.

May also fix an issue introduced
by the recent QSV cleanup patchset.


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6156 b64f7644-9d1e-0410-96f1-a4d463321fa5